1. Field of the Invention
This invention relates to a semiconductor device and a manufacturing method thereof and is applied, for example, to a vertical MOSFET or the like having a trench gate structure.
2. Description of the Related Art
A trench gate structure configured by forming trenches (grooves) in the main surface of a semiconductor substrate and forming gate electrodes by the use of the trenches is applied to a semiconductor device such as an insulated gate bipolar transistor (IGBT) or vertical metal oxide semiconductor field effect transistor (vertical MOSFET), for example. It is mainly used for a power supply (for example, refer to Jpn. Pat. Appln. KOKAI Publication No. 2000-164869) or the like.
The above vertical MOSFET having a trench gate structure has a grater current capacity and lower ON resistance in comparison with the conventional double metal oxide semiconductor (DMOS) transistor and the cost thereof can be expected to be lowered due to chip shrink. Further, since a withstand voltage of approximately several tens of volts to several hundreds of volts can be attained, it can be widely used for a switching power supply of a mobile terminal, personal computer or the like.
However, for example, a power supply system which supplies electric power is desired to perform high-speed and highly efficient operation with an increase in the operation speed of a central processing unit (CPU) of the personal computer or the like. Therefore, in the vertical MOSFET used in the output stage of a DC/DC converter power supply circuit, it becomes particularly important to enhance the switching characteristic thereof with an increase in the operation speed.
In order to enhance the switching characteristic, particularly, it is necessary to reduce the ON resistance and the feedback capacitance between the gate and drain. For example, in an element having a low withstand voltage of 100 V or less, since the proportion of the channel resistance to the ON resistance of the element becomes higher, the ON-resistance tends to become more important.
Next, a conventional semiconductor device is explained by taking the vertical MOSFET as an example. FIG. 30 is a cross-sectional view showing the main portion of a conventional vertical MOSFET. As shown in FIG. 30, a trench 14 is formed deep so that the bottom portion thereof will be formed in an n−-type drain region 11. A gate electrode 15 is filled in the internal portion of the trench 14. Therefore, the length of a channel formed between an n−-type source region 13 and the n−-type drain region 11 is increased and the ON resistance becomes larger. Further, since the area of a portion of the n−-type drain region 11 which faces the gate electrode 15 is increased, the feedback capacitance between the gate and drain is increased. As a result, the Millar charging period at the ON/OFF time becomes longer and it cannot be expected to attain a high-speed switching operation. Therefore, in order to realize a semiconductor device in which the switching characteristic is enhanced, it is desired to reduce the ON resistance and the feedback capacitance between the gate and the drain.